Transmission circuits



.uy 4, 1939. H. NYQUIST' 2,164,752

TRANSMISSION CIRCUITS Fil'ed June 1'7, 1938 lli Patented July 4, 193@ UNITED ST'ESES FJEENT OFFICE TRANSMIS SIGN CIRCUIT S Harry Nyquist, Millburn,

Telephone Laboratories,

N. J., assgnor to Bell Incorporated, New

8 Claims.

This invention relates to transmission systems and more particularly transmission systems in which a number of telephone substations which may be separated from each other by hundreds or even thousands of miles are connected together through one or more interconnecting stations into what is known as a conference system. In such a system each of the participants should be enabled to communicate with the other participants as if they were gathered in conference in a single location. In such a local conference the presence of echoes will not ordinarily interfere with the free interchange of conversation, but in telephone circuits, and particularly in long tele phone circuits, echo effects may tend to become Serious. Such echo eifects are caused by reflection of voice waves whenever electrical irregun larities are encountered in telephone circuits. In a conference system it is particularly difficult to avoid such electrical irregularities and, accordingly, considerable echo effects may arise.

It is the object of this invention to provide efiective means for the suppression of such echoes.

In U. S. patent to A. B. Clark et al. 1,594,454,

,l August 3, 1926, there is disclosed an interconnecting arrangement for several signaling stations which is illustrative of one sort of arrangen ment in which my invention might advantageously be employed.

My invention is an improvement over echo suppressor circuits used heretofore in that lock-out is prevented. Lock-out occurs in the usual conference arrangement by virtue of the fact that the echo Suppressors employed function to give the participant to the conference to speak rst, control of the circuit as long as he continues to talk. Transmission from all other stations is prevented but a second participant starting to speak a small fraction of time later than the iirst may believe himself heard when, as a matter of fact, he is not, or control of the circuit may seesaw back and forth by reason of unintended pauses in conversation. By the arrangement of my invention this condition is avoided and the conditions of a face-to-face conference are closely approximated. I provide, in addition to a master suppressor operated by the presence of a signal at the interconnecting station, and functioning to shut off incoming signals from each of the participant stations, a disabler on each line which functions to disable the master suppressor with respect to that line in the presence on that line of-a signal arising in it. By this arrangement two or more participants may speak simultaneously if they wish just as in a local conference and the composite effects will be audible to all participants. In addition, I may insert a delay device in the connection between the master suppressor and the individual line, said delay device having a delay time nearly equal to the double delay of the particular line so that transmission from that line is not actually cut olf until about the time when transmission coming to the line from the interconnecting station would return again to the interconnecting station as an echo. Thus echoes are prevented from interfering with conversation and at the same time freedom of conversation is permitted.

For a more complete understanding of my invention, reference is made to the drawing in which:

Fig. l is a diagram of a simple conference system;

Fig. 2 shows in simplified form an interconnecting center for a conference system according to the invention; and

Fig. 3 is a diagram of a more elaborate conference system.

In Fig. 1 a conference arrangement is indicated which may be called a simple star connection in which there is a single interconnecting center C. The number of stations participating in the conference may be few or many but each terminates in the single center C. In the spe-- cific illustration of Fig. l there are four branch lines and there is assumed to be a single participating station at the end of each branch I, 2, 3 and l5.

In Fig. 2 which illustrates a specific embcdiment of the invention as applied to a simple star f connection there is shown a cross-connection 20 between the lines of a number of subscribers, in total n, which lines terminate at the interconnecting center shown in the ligure. The diagram is drawn on the basis of a four-wire connection to each subscriber, one pair carrying incoming and the other pair carrying outgoing signals. In the event, as will often happen, that the subscriber connection is by two-wire lines, the subscriber arrangement in the diagram is to be taken to represent that portion of a terminal repeater equipment in which transmission toward and from the interconnecting center is in separate pairs. Such terminal repeaters are often employed in arrangements of this sort, as, for eX- ample, in the specific disclosure of Clark et al., supra. Proceeding from the interconnecting point the resistances 40 prevent reflection and aid line balance at this point, the hybrid transformers H1, H2 and so forth, provide the usual connection between two-wire and four-wire transmission, and the networks N1 and N2 and so forth, provide hybrid coil balance, all in the manner well known in the art. Transmission from the subscriber l (not shown in Fig. 2), for example, towards the interconnecting center is carried by circuit L1 and amplified by amplifier A1 and transmission from the center toward the subscriber i is carried by circuit L1 and amplified by amplier A1'. Transmission to and from the other interconnecting subscribers 2 to n is similarly carried by lines L2 and L2', amplifiers A2 and A2 to lines Ln and Ln and An and An', respectively.

Bridged across the cross-connection. 29 is an amplifier detector or other similar device AD which on the receipt of signal voltage across its input terminal, that is, in the presence at the interconnecting center of a signal which may have had its source at any one oi the interconnected stations, provides operating current to operate Suppressors on the several incoming lines. This is illustrated by the connection from AD' over the multiple connection I9 to the relays R1', R2', and so forth. In the specic embodiment shown, operation of these relays places a short circuit across their respective associated incoming lines L1, L2 and so iorth, thus preventing transmission from the terminal. subscribers station from reaching the interconnecting point. By this means a signal going from the interconnecting center over line L1, for example, to terminal subscriber i and returning as an echo from that terminal over L1 is prevented from reaching the interconnecting center. Since one of these Suppressors is applied to each line all transmission toward the interconnecting center would be prevented and even the signal which energized the amplifier detector AD would be interrupted except that a disabler is associated with each suppressor mechanism. This disabler may take the form of an amplifier detector such as AD1 bridged across line L1 which in the presence of a primary signal in line L1 provides operating current for the relay R1 which, on operation opens normally closed contacts i3 thus disabling the operation of the echo suppressor relay R1 and preventingl or removing the short circuit caused by its sperating to close normally open contacts The impedances 5S isolate the input to ampliiier detector AD1 from the eiTect or" the short circuit caused by relay R1.

In order to provide a maximum freedom from interference with signals from each ci the several participating stations individual delay circuits D1, Dz and so forth, may be inserted in the individual connections between the master suppressor operating amplifier detector AD and the individual short-circuiting relays R1', R2', and so forth. These delay circuits are individually proportioned to insert delay approximately equivalent to the double delay time of the particular subscribers line with which they are associated. Thus, for example, operating current provided by amplifier detector AD as a result of the receipt of a signal at the interconnecting center is delayed in building up to operating value in the relay R1 until a time interval has elapsed which is only slightly less than that required for a signal to travel from the interconnecting center down the line to the terminal of subscriber i and to return as an echo. Thus the relay R1 will operate to suppress an echo returning toward the interconnecting center from the terminal but, on the other hand, the line will remain open for the alaaf/5a transmission of signals arising in it for as large a proportion of the time as possible with proper echo suppression. The greater intensity of a primary signal arising in the line, that is, at the line terminal, will cause the operation of relay R1 by way of amplifier detector AD1 and maintain the line active or, if inactive, immediately restore it to activity. It is obvious that proper operation of this system depends upon a proper interrelation between the functional characteristics of the component apparatus. For eX- ample, the amplier detector AD1 must not be made so sensitive that echo currents will cause the operation of relay R1 and on the other hand operation of the master suppressor circuits through amplifier detector AD must always be preceded by the operation of one or more of the disabler circuits AD1, ADz ADH. These adjustments are diiiicult to specify precisely but are readily made in practice.

It is apparent that by the arrangement described each participant to the conference is enabled to signal, that is, to contribute his part to the conference as freely as if all the participants were gathered together in a single location but that so long as signals do not arise at his station any echoes which arise there are prevented from causing interference to the conference.

The arrangement shown in Fig. 3 may be referred to as a composite star connection. It has a ri lary center PC to which secondary centers SG1, SC2, SC3, are connected by means of trunks. To each one of the secondary senders 'there are branches il, i2, 2i, 22, 3i, 32, and so forth. At the end of each branch there is a party to the conference. The arrangement in this case is to equip the secondary centers with echo suppressors and associated disablers as described and illustrated in Fig, 2. The trunks leading to the primary centers are not equipped with either a suppressor or a disabler and the primary center is not equipped at all, since echoes appear to arise principally at the terminals and any echoes that arise on the circuit are negligible by comparison.

t is to be understood that the particular embodiment described is not to be considered to completely encompass the general principles disclosed. It will be apparent that many other arrangements may be employed within the spirit of the invention as dened by the appended claims.

What is claimed is:

1. A plurality of participating signaling stations, an interconnecting station, transmission circuits connecting said signaling stations to said interconnecting station, signal controlled means at said interconnecting station preventing transmission irom said signaling stations from reaching said interconnecting station, and means associated with the incoming circuit from each of said signaling stations adapted to disable said preventive means with respect to said incoming circuit.

2. in a conference circuit, a plurality of participating signaling-stations, an interconnecting station adapted to enable each of said signaling stations te exchange signals with all other participating stations, individual means associated with each incoming signaling circuit operated by signals at said interconnecting station adapted to short-c' 'cuit said incoming circuit, and local signal-operated means associated with each individual incoming signal circuit adapted to prevent short circuit of said individual circuit.

3. In a conference circuit, a plurality of participating signaling stations, an interconnecting station, means at said interconnecting station enabling a signaling station to exchange signals with every other station, a portion of said enabling means comprising a four-wire circuit between said interconnecting station and said signaling station, a iirst pair of said four-wire circuit adapted for signal transmission from said interconnecting station, a second pair of said fourwire circuit adapted for signal transmission toward said interconnecting station, means to prevent transmission to said interconnecting station from said second pair, signal-operated means at said interconnecting station to effect the operation of said preventive means, and means responsive to the presence of a signal on a particular second pair adapted to clear that particular pair.

4. In a conference circuit according to claim 3, the said signal-responsive means associated with said particular pair adapted to prevent the cperation of said preventive means with respect to that particular pair.

5. In a conference circuit, a plurality of participating signaling stations, an interconnecting station, means enabling the exchange of signals between each signal station and all other signal stations, a transmission line between the interconnecting station and each signaling station, respectively, a four-wire circuit in each respective line between said interconnecting station and a signaling station, a first pair of said four- Wire circuit adapted for transmission from said interconnecting station, the second pair of said four-wire circuit adapted for signal transmission toward said interconnecting station, means for short-circuiting said second pair to prevent transmission to said interconnecting station, signaloperated means at said interconnecting station adapted to effect the operation of said shortcircuiting means, a circuit connecting said signaloperated means with said short-circuiting means, a delay circuit in said connecting circuit, and signal-responsive means associated with said second pair adapted to disable said short-circuiting means with respect to that particular pair.

6. In a conference circuit according to claim 5, the said delay circuit proportioned to have a delay time equal to nearly double the time required for a signal to traverse the line between the said interconnecting station and the respective signaling stations.

7. In a conference system comprising a plurality of signal paths provided with means for sending and receiving signals, with separate circuits for sending and receiving in some portion of said paths, and cross-connected to each other at a common interconnecting point, the method of preventing echoes which consists in shortcircuiting said sending circuits, on paths not sending signals, in response to the presence of signals at said interconnecting point, and disabling said short circuit on a path in which a signal originates.

S. In a conference system comprising a plurality ci signal paths provided with means for sending and receiving signals, with separate circuits for sending and receiving in some portion of said paths, and cross-connected to each other at a common interconnecting point, the method of preventing echoes which consists in short-circuiting said sending circuits, on paths not sending signals, in response to the presence of signals at said interconnecting point, delaying the application of said short circuit withregard to each path respectively for a time nearly equivalent to twice the time required for a signal to traverse that path, and disabling said short circuit on a path in which a signal originates.

HARRY NYQUIST. 

